How to draw a line on a QPixmap using points - c++

Question

I have a label in my GUI that displays an image as a QPixmap. I want to be able to draw a continuous line on my image by simply clicking anywhere on the image to select the start point and then make a second point somewhere else by clicking on a other part of the image. The two points should connect immediately after placing the second point and I want to be able to continue that same line by placing more points on the image.
While I know how to draw something on a QPixmap, the mouse event which I need to use to get the coordinates for the points is really confusing me as I’m still fairly new to Qt.
Any examples for a solution would be much appreciated.

Answer 1

I suggest you to use QGraphicsView for this purpose. Use my code snippet which works perfectly.
Subclass QGraphicsScene:
#ifndef GRAPHICSSCENE_H
#define GRAPHICSSCENE_H
#include <QGraphicsScene>
#include <QPoint>
#include <QMouseEvent>
class GraphicsScene : public QGraphicsScene
{
Q_OBJECT
public:
explicit GraphicsScene(QObject *parent = 0);
signals:
protected:
void mousePressEvent(QGraphicsSceneMouseEvent *mouseEvent);
public slots:
private:
QPolygon pol;
};
#endif // GRAPHICSSCENE_H
.cpp file:
#include "graphicsscene.h"
#include <QDebug>
#include <QGraphicsSceneMouseEvent>
#include <QPainter>
GraphicsScene::GraphicsScene(QObject *parent) :
QGraphicsScene(parent)
{
addPixmap(QPixmap("G:/2/qt.jpg"));//your pixmap here
}
void GraphicsScene::mousePressEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
//qDebug() << "in";
if (mouseEvent->button() == Qt::LeftButton)
{
QPoint pos = mouseEvent->scenePos().toPoint();
pol.append(pos);
if(pol.size() > 1)
{
QPainterPath myPath;
myPath.addPolygon(pol);
addPath(myPath,QPen(Qt::red,2));
}
}
}
Usage:
#include "graphicsscene.h"
//...
GraphicsScene *scene = new GraphicsScene(this);
ui->graphicsView->setScene(scene);
ui->graphicsView->show();
Result:
If you want save new pixmap (or just get pixmap) as image, use this code:
QPixmap pixmap(ui->graphicsView->scene()->sceneRect().size().toSize());
QString filename("example.jpg");
QPainter painter( &pixmap );
painter.setRenderHint(QPainter::Antialiasing);
ui->graphicsView->scene()->render( &painter, pixmap.rect(),pixmap.rect(), Qt::KeepAspectRatio );
painter.end();
pixmap.save(filename);
With render() you can also grab different areas of your scene.
But this code can be better: we create and paint same polygon. If we can remember last painted point, then we can paint line by line (begin of line is end of last line). In this case we don't need all points, we need just last point.
As I promised(Code improvement):just provide additional variable QPoint last; instead of QPolygon pol; and use next code:
void GraphicsScene::mousePressEvent(QGraphicsSceneMouseEvent *mouseEvent)
{
//qDebug() << "in";
if (mouseEvent->button() == Qt::LeftButton)
{
QPoint pos = mouseEvent->scenePos().toPoint();
if(last.isNull())
{
last = pos;
}
else
{
addLine(QLine(last,pos),QPen(Qt::red,2));
last = pos;
}
}
}
As you can see, you store only last point and paint only last line. User can clicks thousands time and now you not need to store this unnecessary points and do this unnecessary repainting.